High speed electrographic engines preferably use roller transfer to move a toner image from a charge retentive dielectric member, such as a photoconductor, to the receiver, which is usually paper or transparency material. Corona wire devices are also used for transferring toner, but the performance of corona transfer is inferior to that of roller transfer, particularly at high speeds. The transfer roller is a conductive, elastomeric roller that is biased to a polarity opposite the toner polarity. During transfer, the front surface of the receiver is brought adjacent the toner image carried by the photoconductor, the roller contacts the back surface of the receiver, and the image is transferred to the front surface of the receiver by the electric field produced by the transfer roller. The area of contact of the transfer roller and the receiver is described as the transfer nip.
An adjustable constant current supply is preferably used to produce a constant charge density on the receiver. This results in a constant electric field for transfer independent of the receiver thickness. Toner with higher charge requires a higher charge density on the receiver and greater transfer current. The electric current setpoint of the constant current supply is adjusted appropriately for high charge toner or low charge toner. Faster process speeds also require proportionally higher current to produce the same surface charge density. At high output currents, the current supply operates at high voltages.
During transfer, as a point on the roller rotates toward the paper, charges must be conducted to the outer surface of the roller. If the roller resistivity is too large or the time interval for approach and passage through to the nip is too short, a high voltage is required for the constant current supply to apply the necessary charge density to the receiver, resulting in a high voltage on the roller surface and producing ionization defects on the image. This is particularly a problem for high toner charge to mass ratios greater than −30 μC/g and at high process speeds greater than 17.5 ips (110 PPM).
Prior art indicates that a minimum surface charge density is required for uniform transfer of toner. High speed processes provide less time for this charge to be applied and for toner transfer. Prior art suggests that increasing the transfer current for high speed processes to apply the appropriate surface charge density will result in uniform toner transfer. However, we found that increasing the current with the prior art transfer roller did not solve the problem for high speed processes without introducing transfer defects, and another solution had to be found.